The invention relates to a microwave resonator for the high temperature treatment of materials. It is intended to permit the sintering or the drying of materials. Such tasks can be optimized with a uniform field distribution within the interior of the microwave resonator.
DE 43 13 806 discloses an apparatus for heating materials by microwaves. The apparatus consists of a heating chamber through which the material to be processed is transported. The heating chamber has a wall portion, which has a concave curvature. The microwave beam coupled into the apparatus is reflected at this wall portion and focussed onto the material volume to be heated.
A similar apparatus is disclosed in WO 90/03714. In this case, the heating chamber is intended to receive food to be heated with a uniform temperature field.
JP 4-137391 discloses a heating chamber with a second reflective wall disposed opposite the first reflective wall. In this way, the process volume is to be filled with a strengthened uniform field to provide for uniform heating of an object in the heating chamber.
U.S. Pat. No. 5,532,462 discloses a cylindrical reaction container whose interior is heated by microwave energy. To this end, a multi-mode microwave is coupled into the container such that it is absorbed at the interior wall and reflected in such a way that the absorption and reflection occurs in a helically progressing fashion. The container interior is said to be uniformly heated in this way.
During the sintering of ceramic parts, inhomogenous field distributions lead to different densities within a single charge and to inhomogeneous densifications within single samples. This, in the end effect, results in mechanical stresses, which may deform shaped parts or which may even shatter them. This problem and the understanding derived therefrom that a uniform volume heating is very important during sintering procedures, that is generally during material processing, are treated in the publication, "MICROWAVE SINTERING OF ZIRCOMA-TOUGHENED ALUMINUMA COMPOSITES" by H. D. Kimreg et al., (Mat. Res. Soc. Symp. Proc. Vol. 189, 1991, Material Research Society, pages 243 to 255). Two high-mode, cylindrical microwave ovens are described, one operated at 2.45 GHz and the other at 28 GHz. The sintering process was successful only at the high frequency.
At the MRS Spring Meeting in San Francisco, Apr. 11, 1996(Symp. Microwave Processing of MaterialsV) L. Feher et al. reported under the title "the MiRa/Thesis 3D-Code Package for Resonator Design and Modeling of Millimeter-Wave Material Processing", about the simulation of the field distribution in a design of a high-mode cylindrical resonator with a spherical cover used by IAP in Nizhny Novgorod.
It is shown therein that resonators with a circular cylindrical or spherical geometry have a field distribution, which generally needs improvement. Because of the topology field focus areas necessarily occur in the interior of the resonator so that only a relatively small operating volume with homogeneous field distribution is available within the resonator volume. Additional technical measures such as mode stirrers and diffuse surfaces, which distribute the microwaves, improve the situation, but they are relatively complicated and expensive for industrial application.
It is the object of the present invention to provide a resonator in which highly inhomogeneous caustic field peaks do not occur. A microwave beam coupled into the resonator should be distributed within the resonator interior so that goods to be heated or to be sintered in the resonator (or microwave oven) are subjected to an essentially homogeneous field.